1. Field of the Invention
The present invention relates to a method and to an apparatus for electroplating a metal, copper for example, onto a substrate. Such method and apparatus may be utilized in the field of electroplating an article to be used as an electrical device, such as a printed circuit board, a chip carrier, including a multichip carrier, or any other carrier having a circuitry thereon.
2. Brief Description of the Related Art
The manufacture of such electrical devices is well-known. The processes for their manufacture comprise a plurality of steps including the metal deposition step for producing the circuitry thereon. These process steps require metallization on the outer surface of the device as well as in the holes or other recesses in the electrical device. For example, printed circuit boards having a plurality of circuitry layers and also having a plurality of holes, namely through holes and blind holes, are to be electroplated with copper in order to generate a copper deposit, the thickness thereof being required to be as uniform as possible. Furthermore, copper deposition in the holes shall also be uniform. Especially, copper deposition is to be consistent both on the outer sides of the electrical device and in its holes to avoid that the holes are not provided with a sufficient copper thickness while coppering on the outer sides has already attained the required deposit thickness.
Pulse plating has mainly proved to be suitable to meet the above objectives. More specifically, reverse pulse plating has been identified to be particularly appropriate. Reverse pulse plating refers to a process comprising applying cathodic and anodic current pulses alternately to the electrical device.
U.S. Pat. No. 6,524,461 B2 for example teaches a method for depositing a continuous layer of a metal onto a substrate having small recesses in its surface. This method comprises applying a modulated reversing electric current comprising pulses that are cathodic with respect to said substrate and pulses that are anodic with respect to said substrate, wherein the on-time of said cathodic pulses is from about 0.83 μs to about 50 ms and the on-time of said anodic pulses is greater than the on-time of said cathodic pulses and ranges from about 42 μs to about 99 ms. In a typical example of the modulated reversing electric current sequence a waveform is used which comprises a cathodic (forward) pulse followed by an anodic (reverse) pulse. An off-period of relaxation period may follow either or both of the cathodic and anodic pulses.
Further, US 2006/0151328 A1 teaches a method of applying a pulse reverse current flow to work pieces having high-aspect ratio holes, i.e., holes whose length is large as compared to the diameter thereof. Holes having an aspect ratio of up to 10:1 and a hole length of 3 mm or even larger shall be processed efficiently. The pulse plating sequence to be applied to the work pieces is taught to comprise cathodic and anodic pulses and is used at a frequency of at most about 6 Hertz. Durations of the forward current pulses and reverse current pulses are indicated to be at least 100 ms (forward) or at least 0.5 ms (reverse), respectively. The peak current density of the forward current pulses is furthermore indicated to be at least 3 A/dm2 and at most 15 A/dm2 and that of the reverse current pulses to be at least 10 A/dm2 and at most 60 A/dm2. In a preferred embodiment of the process described in this document the work pieces are plate-shaped, such as printed circuit boards or any other plate-shaped electrical circuit carriers. In this preferred embodiment, the method comprises (a) applying a voltage to between a first side of the work piece and at least one first anode, to the effect that a first pulse reverse current flow is provided to the first side of the work piece, wherein said first pulse reverse current flow has at least one first forward current pulse and at least one first reverse current pulse flowing in each cycle time, and (b) applying a second voltage to between a second side of the work piece and at least one second anode, to the effect that a second pulse reverse current flow is provided to the second side of the work piece, wherein said second pulse reverse current flow has at least one second forward current pulse and at least one second reverse current pulse flowing in each cycle time. In a particularly preferred embodiment the first forward and reverse current pulses of one cycle are offset relative to the second forward and reverse current pulses of one cycle, respectively. This offset may advantageously be approximately 180°. It is furthermore indicated that, for further improving throwing power, the current flow may comprise, in each cycle time, one forward current pulse followed by one reverse current pulse and after that one zero current break.
It has proved that the above method is particularly useful in achieving good throwing power of copper deposition, i.e., uniform copper layer formation on the outer side of the work piece and on the walls of holes contained therein.
Such objective is however, not achievable with the plating conditions described, if the substrate to be plated is provided both with regions where there are many holes per unit area on the one hand and with regions where there are no or only a few holes per unit area on the other hand. Using the method described in US 2006/0151328 A1 will not consistently metalize these regions: In those regions, where no or only a few holes are provided, copper thickness will be large as compared to those regions which have many holes per unit area.
Furthermore, it has proved disadvantageous that this known method of hole wall plating leads to differing coppering results in the holes in different regions of a board.
It has proved necessary that through holes are first X- (bridge-) plated, i.e., deposition shall lead to enhanced copper deposition in the middle of the hole thereby closing it by forming a copper plug there, thus forming two blind holes each one being accessible from one of the sides of the board. Then the two hole parts are completely filled which means that the total volume of the hole is filled with metal. When a known method is used to perform this procedure, holes being located in the border area of the board will not be plated as efficiently as holes being located in the middle thereof. Consequently, the border holes will not be closed in their center region when the middle holes are filled already. This leads to an undesirable situation wherein hole-filling is varying in the different regions of the board.
Furthermore, conformal plating of through holes and blind holes, i.e., plating of a thin layer of copper on the walls of the holes without filling these, is not uniform when the method of US 2006/0151328 A1 is used.